Bicycle frames may be equipped with no suspension, only a front suspension, or with both a front and rear suspension. Bicycles with only front suspension are referred to as hardtail bicycles. Bicycles with only a rear suspension are fairly uncommon, as bicycles with a rear suspension generally also have a front suspension. Bicycles with suspension in both the front and rear of the frame are referred to as full-suspension bikes.
In the distant past, bicycles often had no suspension other than the flex of the frame and softness of the pneumatic tires. Thus, the bicycles were essentially “rigid,” such that the bicycle frame transmitted shock from riding on uneven terrain directly to the rider through the seat, pedals, and handlebars. The transmitted shock through the rigid frame to the rider resulted in discomfort to the rider. It also contributes to loss of control, especially with a mountain bike on rough terrain.
The most common frame design for a conventional upright bicycle consists of two triangles (i.e., a “diamond frame”), a main triangle forming the front of the frame and a paired triangle forming the rear of the frame. In the diamond frame, the main “triangle” consists of four tubes: the head tube, top tube, down tube and seat tube. The head tube contains the headset, which is the interface with the front forks and the handlebars—allowing them to rotate together relative to the frame. The top tube connects the head tube to the seat tube at the top of the frame. The down tube connects the head tube to the bottom bracket shell. The bottom bracket shell is located at the bottom of the seat tube. A bottom bracket is housed within the bottom bracket shell. The bottom bracket on a bicycle includes a crankset, which rotates. The bottom bracket comprises a spindle that houses the crankset and bearings, which allow the crankset to rotate relative to the spindle. The crankset is the component of a bicycle drivetrain that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain, which in turn drives the rear wheel. The crankset consists of one or more sprockets, also called chainrings attached to cranks to which the pedals attach.
The rear triangle consists of the seat tube connected by paired chain stays at the bottom of the seat tube and seat stays at an upper portion of the seat tube. The rear triangle connects to the rear fork ends or rear dropouts, where the rear wheel is attached. The rear fork ends include slots or apertures in the bicycle frame on each side of the rear wheel where the axle of a bicycle wheel is attached. The chain stays run connecting the bottom bracket to the rear fork ends. The seat stays connect the top of the seat tube, often located at or near the same point as the top tube, to the rear fork ends.
In order to reduce the discomfort associated with bicycles ridden on uneven terrain, suspension systems were developed. Besides providing comfort to the rider, suspension systems improve traction and safety by helping to keep one or both wheels in contact with the ground. Front suspension systems are often implemented using a telescopic fork configured as a shock absorber with a spring and damper in order to reduce transmitted shock. The implementation of the front suspension systems is relatively uncomplicated as the front wheel is supported by left and right forks that attach to a single head tube at the front of the bicycle frame. Rear suspension system designs may be more complicated due to the fact that the rear wheel is typically supported both by substantially horizontally oriented chain stays that are connected to the bottom bracket shell below the seat post and frame stays, which extend from the rear fork ends to an upper portion of the seat tube.
Rear bicycle frame suspensions are most commonly found in mountain bikes, due to the rough terrain experienced by mountain bike riders. The rear suspension, as with the front suspension, allows the rear wheel to move up and down relative to the rider to absorb shock forces and also to improve tire contact with the ground in rough terrain. Many early rear suspension designs included a shock absorber located between the rear frame and the seat post mounted in varying ways. Some rear suspensions also modified the conventional rear frame designs by utilizing additional frame members, or linking members between the conventional rear frame with the chain stays at the bottom of the seat tube and seat stays at an upper portion of the seat tube and the seat post. Some rear suspensions also modified the configuration of the seat tube.
The single pivot is the simplest type of rear suspension. The rear axle is held by a swingarm which is connected to the frame via a single pivot located on the bottom tube near the bottom bracket shell. When the suspension moves through its travel, the path the rear axle describes is a circle around the single pivot point. The rear triangle may simply be the swingarm. The rear shock may also be attached between the seat tube and the swingarm. This configuration of the single pivot design allows for a fairly linear leverage ratio between wheel travel and shock absorber travel. The main advantage of the single pivot design is its simplicity. It has few moving parts, few pivot points, is relatively easy to design and has good small bump compliance. Challenges with this single pivot design are brake jack and chain growth. Pedal induced forces and rider bobbing can rob the system of efficiency. Due to the limited power output of a human on a bicycle any lack of efficiency is undesirable. Some of these designs are heavy and tend to bounce up and down while a rider pedals. This movement takes power out of a rider's pedal stroke, especially during climbs.
Input from hard braking efforts also negatively affects full suspension designs. When a rider applies the brakes, some of these suspensions compress into their travel and lose some of their ability to absorb bumps. This may happen in situations where the rear suspension is needed most. When braking efforts cause the suspension to compress it is referred to as brake squat, when braking causes the suspension to extend it is called brake jack.
Some rear suspensions were designed to overcome or mitigate the above problems. One variation of single pivot suspension places the pivot in front of and above the bottom bracket, at a height above the smallest chain ring or higher. This gives the design a significant amount of anti-squat when pedaling in smaller chain rings, which helps reduce loss of energy due to squat. This is particularly of importance on steep climbs, when one would use the smaller chain rings. However, this is a trade-off since the pivot's placement causes the design to suffer more from pedal kickback.
Another variation on the single pivot design is the split pivot design. The split pivot design is a special case of linkage driven single pivot in which one of the four-bar's pivot points coincides with the rear axle. This allows for the disc brake caliper to be mounted on the floating linkage instead of on the swingarm. As a result of this the braking torque now interacts with the suspension via the floating linkage. The linkages can be designed such that this has a positive effect on suspension performance under braking, typically reducing brake jack. Furthermore, the relative rotation between brake disc and brake caliper as the suspension goes through its travel is different from that in single pivot designs. The four linkages in a split pivot design influence how braking torque is transmitted, how the brake caliper moves in relation to the disc and influence the leverage ratio between wheel travel and shock travel. Since these influences may have a different optimum linkage design, the bike's design has to strike a balance.
Other suspension designs rely on the shock absorber design to reduce pedal-induced bobbing and squat. There are many additional rear suspension designs in the prior art. However, a need exists in the art for further improvements of bicycle rear suspension systems to further improve the performance of the rear suspension under braking, acceleration, and further improved anti-squat anti-dive characteristics without complicating trade-offs in performance.